Many lubricating oil compositions for internal combustion engines have been proposed in the past. For example, JP 2003-073685 discloses a lubricating oil composition for internal combustion engines, which exhibits excellent abrasion resistance and high temperature cleaning properties.
In order to improve fuel economy, it is important to prevent energy losses caused by friction inside an engine when an automobile is running. That is, reducing the coefficient of friction of sliding parts by adding a friction modifier, as disclosed in JP 2003-073685, is effective for improving fuel economy. In addition, adding a viscosity index improving agent is effective for lowering kinematic viscosity at low temperatures while maintaining kinematic viscosity at high temperatures and for lowering viscous drag at low oil temperatures.
However, friction modifiers and polymethacrylate-based viscosity index improving agents, which exhibit a significant viscosity index improvement effect, readily undergo thermal decomposition, adversely effect the cleaning properties of engine oils and are thought to hasten the generation of sludge. In particular, there are concerns that viscosity index improving agents and viscosity modifiers will undergo thermal decomposition and cause a build-up of sludge around piston rings and at piston under crowns, which are exposed to high temperatures. In particular, if sludge around a piston rings causes the piston ring to stick, it is not possible to reliably seal in combustion gases by means of the cylinder and the piston ring, and this leads to a deterioration in fuel economy and abnormal wearing between the cylinder and the ring. In addition, if sludge builds up at an under crown, thermal conductivity deteriorates, heat from the combustion chamber cannot escape, abnormal thermal expansion occurs due to high temperatures, and piston cracking and so on can occur.
Therefore, in order to suppress oil oxidation, suppress the generation of sludge and increase engine protection performance, it is important for an oil to exhibit excellent oxidation stability and cleaning properties. Boron-modified dispersing agents are used to improve cleaning properties at high temperatures.
However, the use of boron-modified dispersing agents is not effective for lowering friction, leads to a deterioration in thermal and oxidation stability and corrosion of metals, and leads to an increase in the acid value of an oil and corrosion of non-ferrous metals.
Therefore, the present invention addresses the problem of providing a lubricating oil composition for internal combustion engines, which exhibits a lower coefficient of friction and improved cleaning properties without causing a deterioration in thermal and oxidation stability and corrosion of metals.